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Retirement Benefit Plans - Movements in Present Value of Defined Benefit Obligation (Detail) - TWD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Remeasurement:      
Actuarial loss (gain) arising from experience adjustments $ 94.3 $ 494.1 $ (438.0)
Actuarial gain arising from changes in demographic assumptions 277.4   (233.2)
Actuarial loss arising from changes in financial assumptions (540.5) 3,161.9 541.7
Present value of defined benefit obligation [member]      
Disclosure of defined benefit plans [line items]      
Balance, beginning of year 16,980.3 13,484.1 13,662.7
Current service cost 145.3 123.3 135.6
Interest expense 66.7 118.8 175.4
Remeasurement:      
Actuarial loss (gain) arising from experience adjustments 94.3 494.1 (438.0)
Actuarial gain arising from changes in demographic assumptions 277.4   (233.2)
Actuarial loss arising from changes in financial assumptions (540.5) 3,161.9 541.7
Benefits paid from plan assets (431.8) (399.0) (344.1)
Benefits paid directly by the Company (6.2) (2.9) (16.0)
Balance, end of year $ 16,585.5 $ 16,980.3 $ 13,484.1